Typical "two-sided cling" wrap has an A/B/A structure wherein the A layer provides the cling property and the core B layer provides toughness, support and strength. "Single-sided cling" wraps have either an A/B or an A/B/C structure wherein the A layer provides the cling property, the core B layer provides toughness and strength and the C layer is substantially cling free. These overwrap materials or films are used, among others, in the unitization of pallet loads of individual packages. Ideally, these overwrap materials or films should be of A/B or A/B/C structure, should have high strength, be resistant to tearing and puncturing, and exhibit single-sided cling properties. These single-sided cling properties are necessary to firmly unitize the pallet load while at the same time preventing individual wrapped pallets from clinging to each other with resultant tearing of the film when the pallets are separated from each other.
Several techniques have been developed for achieving cling. These include corona discharge treatment of one side of a film to develop cling on the treated side; laminating or co-extruding at least two films --one having a cling property with the other being non-cling and providing strength; and adding tackifiers to the film composition. Thus, for example, U.S. Pat. Nos. 4,399,180 and 4,418,114 to Briggs et al. relate to a stretch wrap including three layers; a thin skin which imparts cling and gloss properties and a thicker core which provides tear and puncture resistance as well as toughness. The skin layer is a "highly branched" low density polyethylene while the core includes a linear low density polyethylene copolymer with a C.sub.4 -C.sub.10 .alpha.-olefin.
Some development work has focused on the addition of a tackifier to the skin composition in order to enhance cling properties. For example, U.S. Pat. No. 4,425,268 to Cooper is directed to a film which includes 0.5-6 wt.% of a tackifier, such as low molecular weight polyisobutene, amorphous polypropylene, polyterpene or microcrystalline wax. The film also includes 40-90 wt.% of a high molecular weight ethylene vinyl acetate containing 4-25% vinyl acetate and having a melt index of 0.1-4.0and 8-55 wt.% of a copolymer of ethylene and an olefin.
U.S. Pat. No. 4,542,188 to van der Heijden relates to a film having one side of higher cling than the other side without using corona discharge treatment. The film includes two polymers and a tackifier. One polymer, comprising 65-80 wt.% of the film, is a crystalline polymer having low or no cling. The other polymer, comprising 20-35 wt.% of the film, is a linear low density copolymer of ethylene and an .alpha.-olefin having more than three carbon atoms. The film includes 2-10 wt.% of a tackifier, polybutylene, with a molecular weight of less than 2,000.
U.S. Pat. No. 4,430,457 to Dobreski is directed to a film comprising 90-97.5 wt.% of an ethylene polymer. The ethylene polymer may be selected from the ethylene vinyl acetate copolymers containing 0.3-15 wt.% vinyl acetate, linear low density polyethylene resins and low density polyethylene homopolymers. Further, the film includes 2.5-10 wt.% of an amorphous low molecular weight ethylene propylene copolymer having a ethylene content of between 1 and 7 wt.%. It is further specified that the ratio of ethylene polymer viscosity to ethylene propylene polymer viscosity should be in the range 100 to 100,000.
U.S. Pat. No. 4,170,403 to Huke is directed to a film comprising a blend of polyethylene and 0.5-10 wt.% polyisobutylene. One side of the film is treated with a corona discharge to increase its adhesiveness. Thus, one side of the film has a higher cling property than the other.
U.S. Pat. No. 3,932,563 to Argurio et al is directed to a "self-sealing" film or cling wrap film. The composition of this film includes 90-99.5 wt.% of a copolymer of ethylene and a vinyl ester of a C.sub.1 -C.sub.30 monocarboxylic acid. Further, the film composition includes either 0.05-10 wt.% of a hydrocarbon resin or 0.05-5 wt.% polyisobutylene or polybutene. It is further specified that the sum of the weights of the hydrocarbon resin and polyisobutylene or polybutene should be less than 10 wt.%. Finally, the film includes 0.1 to 3.0 wt.% of a partial carboxylic acid of a polyol.
While the above methods of treatment and additives provide some enhancement of the cling property, there yet exists a need for a film with an even greater cling property that can be produced on conventional coextrusion equipment.